Conventional internal combustion engines utilize a slider-crank mechanism to convert the reciprocating motion of a piston into the rotating motion of a crankshaft. The piston reciprocates within a cylinder bore, which is closed on one end by a cylinder head or other structure. The reciprocating piston motion defines a changing volume, which is minimized when the piston is closest to the cylinder head and maximized with the piston is furthest from the cylinder head. The volume changes from minimum to maximum to minimum once per revolution of the crankshaft. Each minimum to maximum event is called a stroke. In a four stroke engine, air for combustion is drawn into the volume during the first stroke. Torque is applied from the crankshaft to compress the combustion air during the second stroke. Fuel is added and is converted into heat and pressure in the combustion chamber when the volume is near its minimum. The heat and pressure act on the piston creating torque on the crankshaft during the third stroke. The magnitude of the torque varies as the mechanical advantage of the slider-crank changes and, more importantly, as the pressure dissipates due to the increased volume created by the piston motion. Exhaust gas exits the volume during the fourth stroke. The four strokes require two full rotations of the crankshaft. The torque on the crankshaft from one cylinder's four strokes is fairly neutral during the fourth and first strokes, somewhat negative during the second stroke, and strongly positive during the third stroke. This torque pulse manifests in both a rolling motion of the engine, which must be managed by engine mounts, and an output torque that oscillates about its mean, which must be managed by the driveline.
The number of combustion events per revolution of the crankshaft determines the primary order of the oscillating torque output and its corresponding oscillating engine mount motion. A four cylinder four stroke engine has a second order torque output; a six cylinder four stroke engine's torque output is primarily third order. Higher order engines have lower peak displacement in the engine mounts and lower peak instantaneous torque, hence less noise, vibration, and harshness (NVH).
The number of cylinders in an internal combustion engine plays a significant role in determining the engine's friction and heat rejection characteristics. For a given displacement, fewer cylinders will generally result in better thermal efficiency and lower friction, both of which translate into improved fuel economy. Thus, it is desirable to have engines with a low number of cylinders, or low cylinder-count engines.
Unfortunately, a low cylinder-count engine has a low primary torque order which manifests in NVH challenges in both the engine mounts and the driveline. Mount and driveline vibration of engines with higher cylinder counts are less because the torque pulses from the cylinders occur at a higher frequency and overlap with each other, which lowers the NVH. Low cylinder-count engines, on the other hand, have a lower torque pulse frequency and a higher torque pulse magnitude, which generates an unacceptable NVH.
Technologies exist to mitigate the effects of displacement on engine mounts, but most can only be tuned to manage one or a few frequencies. Said technologies cannot mitigate the firing order over the whole engine operating speed range. Said technologies are typically applied to the lowest resonance frequency of the system. The remaining frequencies generated through the full operating range of the engine remain problematic for engine torque pulsation in low cylinder-count engines. Because engine torque pulsation frequency varies with engine speed, engine mount displacement mitigating solutions that are designed for a single frequency do not adequately compensate for the torque pulsations that can occur over a wide range of engine speeds.
Additionally, the driveline, including the transmission, driveshaft, differential, and axels, of a vehicle is affected by vibrations. As with engine mounts, technologies exist to manage driveline vibrations caused by torque pulses. For example, dual mass flywheels have been used as an effective method for mitigating driveline vibrations of a specific frequency. However, such frequency mitigating technologies are applied to the driveline's lowest resonance frequency in the operating range as a means to control the worst case vibration condition. Accordingly, vibrations at other frequencies within the operating speed range are less effectively mitigated by known technologies, such as dual mass flywheels. For these reasons, the inability to manage torque pulsation has limited the desirability of high efficiency, low cylinder-count engines.